add condition variable examples

src/ch03/cond_cpp.cpp

@@ -0,0 +1,82 @@
+
+#include "errors.hpp"
+
+#include <chrono>
+#include <condition_variable>
+#include <iostream>
+#include <mutex>
+#include <thread>
+
+using ClockType    = std::chrono::high_resolution_clock;
+using DurationType = std::chrono::duration<double, std::milli>;
+
+struct my_struct_tag {
+  std::mutex mutex;             /* Protects access to value */
+  std::condition_variable cond; /* Signals change to value */
+  int value = 0;                /* Access protected by mutex */
+};
+
+using my_struct_t = my_struct_tag;
+
+my_struct_t data;
+
+DurationType hibernation(1000.0); /* Default to 1 second */
+
+/*
+ * Thread start routine. It will set the main thread's predicate
+ * and signal the condition variable.
+ */
+void
+wait_thread()
+{
+  int status;
+
+  std::this_thread::sleep_for(hibernation);
+  std::lock_guard<std::mutex> lk(data.mutex);
+
+  data.value = 1; /* Set predicate */
+  data.cond.notify_one();
+}
+
+int
+main(int argc, char* argv[])
+{
+  /*
+   * If an argument is specified, interpret it as the number
+   * of seconds for wait_thread to sleep before signaling the
+   * condition variable.  You can play with this to see the
+   * condition wait below time out or wake normally.
+   */
+  if (argc > 1) {
+    int xHibernation = atoi(argv[1]);
+    hibernation      = DurationType(1000.0 * xHibernation);
+  }
+
+  /*
+   * Create wait_thread.
+   */
+  std::thread t(wait_thread);
+
+  /*
+   * Wait on the condition variable for 2 seconds, or until
+   * signaled by the wait_thread. Normally, wait_thread
+   * should signal. If you raise "hibernation" above 2
+   * seconds, it will time out.
+   */
+  auto timeout = ClockType::now() + std::chrono::seconds(2);
+
+  {
+    std::unique_lock<std::mutex> lk(data.mutex);
+
+    if (!data.cond.wait_until(lk, timeout, []() { return data.value != 0; })) {
+      std::cout << "Condition wait timed out.\n";
+    }
+
+    if (data.value != 0)
+      std::cout << "Condition was signaled.\n";
+  }
+
+  t.join();
+
+  return 0;
+}